1. Field of the Invention
The present invention relates to a shower head structure mounted in a film forming apparatus for using a process gas to form a film on a material to be treated, such as a semiconductor wafer, and a cleaning method of the structure.
2. Description of the Related Art
In a process of manufacturing a general semiconductor integrated circuit, in order to form a wiring pattern or fill a concave portion a recess provided between wires made metal such as tungsten (W), titanium (Ti), or copper (Cu), or metal compound such as tungsten silicide (WSi), titanium nitride (TiN), titanium silicide (TiSi), or tantalum oxide (Ta2O5) are deposited to form a thin film on the surface of a to-be-freated semiconductor wafer.
As methods of forming the thin metal film, three methods are known: a hydrogen (H2) reducing method; a silane (SiH4) reducing method; and a dichlorosilane (SiH2Cl2) reducing method. In these methods, the SiH2Cl2 reducing method comprises using, for example, dichlorosilane as a reduction gas to form a W or tungsten silicide (WSi) film at a high temperature of about 600° C. in order to form the wiring pattern. Moreover, similarly, to form the wiring pattern, the SiH4 reducing method comprises using, for example, silane as the reduction gas to form a W or WSi film at a temperature of 450° C. or less than the temperature of the SiH2Cl2 reducing method. Furthermore, in order to fill a hole or the concave portion a recess provided between wires and flatten the surface of the wafer, the H2 reducing method comprises using, for example, hydrogen as the reduction gas is used to deposit the W film at a temperature of about 380 to 430° C.
Additionally, a reducing method constituted by appropriately combining the aforementioned methods is also known and, for example, tungsten hexafluoride (WF6) is used in these methods.
FIG. 9 shows a constitution example of a general film forming apparatus for forming the aforementioned thin metal film. Moreover, FIG. 10 is an enlarged view showing a shower head structure of FIG. 9 in detail.
Aluminum, or the like, is used to form the cylindrical shape of the process chamber 2. A susceptor 4 formed of a thin carbon material or an aluminum compound is disposed in the process chamber 2, and a heater 8 such as a halogen lamp is disposed under the base via a transmission window 6 formed of quartz.
A semiconductor wafer W carried from the outside is laid on the susceptor 4, and a peripheral edge of the wafer W is pressed by a clamp ring 10 constituted such that the ring can be raised/lowered, and fixed onto the susceptor 4. A shower head structure 12 formed, for example, of aluminum is disposed opposite to and above the susceptor 4. A large number of gas ejecting holes 14 are substantially uniformly arranged/formed on a lower surface of the shower head structure 12.
Moreover, for the shower head structure 12, in order to maintain a temperature to be steadily low to some degree during a film forming process, a heat transfer medium 16 (e.g., a Chiller (tradename)), for example, of about 50° C. is passed inside. The shower head structure 12 has a head main body 7, and the body is attached to a chamber ceiling portion 2a via a bolt 5 as shown in FIG. 10. An injection plate 11 with a large number of injection holes 9 formed therein is attached to the lower surface of the head main body 7 via a bolt 13.
A diffusion plate 17 with a large number of diffusion holes 15 formed therein is disposed in a space inside the head main body 7, so that gas introduced into the head main body 7 is diffused in the direction of a wafer surface. A shower base channel 18 is disposed in a side-wall portion of the head main body 7, and the heat transfer medium 16 is passed through the channel. Moreover, during the film forming process, the susceptor 4 is irradiated with heat rays from the heater 8 through the transmission window 6, and the semiconductor wafer W fixed onto the susceptor 4 is indirectly heated to obtain a predetermined temperature.
Additionally, when WF6 or H2 is uniformly supplied as the process gas onto the wafer surface from the gas injection hole 14 disposed above the susceptor 4, a metal film of tungsten is formed on the surface of the wafer.
The aforementioned film forming process comprises continuously processing a plurality of, for example, 25 wafers, one by one, into a film in a sheeting manner, and using ClF3 or another cleaning gas to perform dry cleaning (flushing) for the purpose of removing excess film attached to the member in the process chamber 2, such as the base, clamp ring and shower head structure, during continuous film formation. In this manner, in general, the continuous film forming process and cleaning process are repeatedly performed over a plurality of wafers.
Additionally, in order to maintain electric properties, and the like of the deposited film formed on each wafer to be constant as designed, reproducibility needs to be maintained high so that the thickness of the film deposited on each wafer remains substantially constant. However, in practice, the film thickness obtained by performing the film forming process on a first wafer immediately after a cleaning process may be considerably different from that of the 25-th wafer, during continuous processing of 25 wafers. For example, when the number of wafers in a continuous processing is increased, the thickness of the film formed on the wafer tends to gradually decrease. Although the heat transfer medium 16 is passed through the shower base channel 18 of the shower head structure 12 during idling of the process chamber 2, temperature rises. Moreover, when the process gas is passed from the shower head structure 12 and the number of treated wafers increases, the temperature of the shower head structure 12, particularly the temperature of the gas injection plate gradually drops and settles at a desired temperature.
To solve the problem, the temperature of the gas injection plate of the shower head structure 12 is maintained to be low beforehand, and the continuous film forming process of the wafer is performed. However, when the gas injection plate of the shower head structure 12 is maintained at a relatively low temperature from the start of the processing, compounds relatively difficult to remove in the cleaning process, such as titanium fluoride (TiFx) and other reactive byproducts are attached to the surface of the gas injection plate. The problem below newly arises. When some titanium atoms in a titanium containing film, such as a titanium metal film or titanium nitride film, already deposited on the surface of the wafer in the previous process react with fluorine of WF6 gas supplied during the film forming process, titanium flouride is generated.
FIGS. 11A, 11B show a relation between the temperature of the gas injection plate of the shower head structure and the film thickness of the reactive byproduct attached to the shower head structure. FIG. 11A shows a characteristic of continuous processing of 25 wafers on tungsten metal films with a film thickness of 100 nm at a susceptor temperature of 410° C., and FIG. 11B shows the characteristic of the continuous processing of 25 wafers on tungsten metal films with a film thickness of 800 nm at a base temperature of 460° C.
As is apparent from these drawings, when the temperature of the gas injection plate of the shower head structure drops to about 80° C. from about 100° C., the film thickness of the reactive byproduct rapidly increases. Moreover, a general cleaning process comprises maintaining the temperature of the structure in the process chamber 2 to be substantially the same as the temperature in the film forming process, and passing a cleaning gas such as ClF3 gas. In this case, an unwanted tungsten film attached to the susceptor 4, and the like is removed, but there is a problem that the reactive byproducts, such as titanium fluoride attached to the gas injection plate surface of the shower head structure, are not easily removed.
Moreover, there is a method of setting the temperature of the gas injection plate of the shower head structure during the film forming process to be relatively high, to prevent the reactive byproduct from being attached. However, depending upon the film forming conditions, the maximum temperature of the gas injection plate of the shower head structure limits the amount of byproducts that can be removed as they are best removed at temperatures above this. In this case, the amount of reactive byproducts continues to rise, thus limiting the number of wafers that can be treated in one batch, and a cleaning process is required in accordance with an amount of attached byproducts.